7.1 - 7.3 - Natural Selection

Charles Darwin

• Research: Most notable work done at the Galapagos Islands

• Observations:

  • Geographical Influence: Life forms varied based on geography.

  • Finches: Different shapes of beaks related to food sources on the islands.

  • Tortoises: Variations in shell shapes and sizes based on diet and the ability to reach certain plants.

Evolution

• Definition: Darwin referred to it as “descent with modification,” which is the change in the genetic makeup of a population over time driven by natural selection.

• Evidence: Supported by multiple lines of scientific evidence, making it a robust theory.

Natural Selection

• Foundational Observations by Darwin:

  • Heritability of Traits: Traits are passed from parents to offspring.

  • Adaptations: Inherited characteristics that enhance an organism's survival and reproduction.

• Key Concepts:

  • Adaptations: Inherited traits that improve an organism's fitness in a particular environment. For example, Arctic foxes have large ears to help with temperature regulation, and camels have long eyelashes to protect against sand.

  • Fitness: An organism’s ability to survive and reproduce. Measured by reproductive success.

  • Selective Pressure: Changes in the environment exert selective pressures, influencing which traits are advantageous.

Examples of Natural Selection

• Brown vs. Green Beetles: Brown beetles might have higher fitness compared to green beetles if brown coloration provides better camouflage from predators, leading to more offspring.

• Environmental Changes:

  • Mice Fur Color: Changes due to volcanic eruptions altering the landscape.

  • Peppered Moths: Industrial revolution led to changes in moth coloration due to soot on trees.

• Resistance Evolution:

  • Antibiotic Resistance: Bacteria that survive antibiotic treatment reproduce, increasing the frequency of resistance genes.

  • Pesticide Resistance: Similar process in insects where those resistant to pesticides survive and reproduce.

Key Principles of Natural Selection

1. Overproduction of Offspring:

   • More offspring are produced than can survive, leading to competition for limited resources.

   • This competition results in differential survival, where some individuals are more fit and have better chances of surviving and reproducing.

2. Accumulation of Favorable Traits:

   • Traits that enhance survival and reproduction, known as “favorable” traits, accumulate in the population over generations.

   • Population Evolution: It’s important to note that populations evolve, not individuals.

Artificial Selection

Definition: The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.

• Examples:

  • Dogs: Different breeds have been developed from wild wolves through selective breeding.

  • Plants: Crops have been modified to enhance yield, taste, and resistance to pests.

Comparison with Natural Selection:

• Natural Selection: Nature selects traits that are better suited for survival and reproduction.

• Artificial Selection: Humans select traits that are desirable for domestication or agriculture.

• Common Outcome: Both lead to evolutionary changes in organisms, but natural selection occurs naturally, without human intervention.

Evolution and Populations

Evolution: The change in the genetic makeup of a population over time.

• Population: A group of individuals of the same species living in the same area, capable of interbreeding to produce fertile offspring.

Population Genetics

Definition: The study of the genetic composition of populations and how it changes over time.

• Gene Pool: The total genetic diversity found within a population, consisting of all copies of every type of allele.

• Measuring Genetic Composition: For any given gene, researchers measure the frequency of dominant versus recessive alleles in the population.

Impact of Random Events on Genetic Makeup

1. Mutations:

   • Definition: Random changes in DNA that can introduce genetic variation.

   • New Alleles: Mutations can create new alleles, contributing to genetic diversity.

   • Mutation Rates:

     • Plants and Animals: Tend to have slower mutation rates due to longer generation times and more complex regulation.

     • Prokaryotes: Have faster mutation rates because they reproduce quickly and have simpler genetic regulation mechanisms.